Work on the structure and function of the contractile proteins of muscle is in progress. We have recently isolated the native thick filaments of vertebrate striated muscle to investigate the subunit structure, composition, and the possible movement of cross-bridges in the presence of metabolites such as ATP and Ca2 ion. We find four myosin molecules per 143 A repeat in the thick filament based on two independent determinations. The number of C-protein molecules associated with each thick filament has also been evaluated. Paired sedimentation studies of the filaments in the presence and absence of calcium ion reveal structural changes within the filaments induced by Ca2 ion at concentrations of this divalent metal comparable to that required for muscle activation (pCa equals 5.5). We have also initiated an investigation of the role of two essential sulfhydryl groups (SH1 and SH2) in the mechanism of ATP cleavage by myosin in the presence of mM Mg 2 ion. These studies provide evidence for the formation of an inhibiting cyclic ternary structure involving the two essential sulfhydryls and the MgATP substrate. Chemical blocking by either group prevents the formation of the complex and allows rapid hydrolysis to occur. Our studies suggest that actin acts in a manner similar to chemical modification in that it breaks the inhibiting complex by interacting at or near the SH1 group, thus allowing a large increase in the ATP hydrolysis rate.